1. Field of the Invention
The present invention relates generally to image forming apparatuses and image forming methods.
2. Description of the Related Art
In an image forming apparatus, a bias opposite in polarity to a bias necessary for printing is typically applied to perform cleaning. There is a known technique for obtaining such a cleaning bias without a circuit dedicated only for this purpose by applying biases of a same polarity output from a single transformer to rollers facing each other. An example of such a technique is disclosed in Japanese Laid-open Patent Publication No. 2009-122168.
An image forming apparatus disclosed in Japanese Laid-open Patent Publication No. 2009-122168 performs image formation by: forming a toner image by causing toner to stick to an electrostatic latent image on a photosensitive element; transferring the toner image onto an image carrier using a primary transfer roller; performing secondary transfer of transferring the toner image from the image carrier onto a medium, such as paper, using a secondary transfer roller; and fixing the toner image onto the medium. Secondary transfer biases that allow supplying a required amount (or value) of electric current for the secondary transfer are applied to the image carrier and the secondary transfer roller.
Japanese Laid-open Patent Publication No. 2009-122168 also discloses another example, in which secondary transfer biases that are opposite in polarity are applied to the facing rollers.
In an image forming apparatus that employs an intermediate transfer system, toner is caused to stick to electrostatic latent images on one or more photosensitive elements (e.g., four photosensitive elements for yellow, magenta, cyan and black or a single photosensitive element for shared use among yellow, magenta, cyan, and black) for respective employed colors. The toner images are temporarily transferred onto another image carrier (e.g., a transfer belt). The transferred toner images are transferred a second time (hereinafter, “secondary transfer”) onto a to-be-printed medium (hereinafter, “medium”), such as paper.
The secondary transfer is performed by applying an electrical charge that exerts an attractive force or a repulsive force to a charge of the toner, thereby transferring the toner image from the transfer belt onto the medium.
For instance, in a case where the toner is negatively charged, a secondary transfer bias (negative bias) may be applied to an intermediate transfer driving roller so that a repulsive force transfers the toner image from the transfer belt to paper. Alternatively, a secondary transfer bias (positive bias) may be applied to the secondary transfer roller so that an attractive force transfers the toner image from the transfer belt onto paper.
Because the toner image is transferred by action of the charge, it is necessary to supply a predetermined value of electric current. The required amount of charges (i.e., the value of electric current) depends on an amount of the toner (more specifically, an image to be printed). For this reason, a scheme that transfers toner from a photosensitive element to a transfer belt by applying a constant voltage bias is adopted by a number of example configurations. In this scheme, application of a desired value of electric current is achieved by changing the value of electric current in accordance with an image to be printed. As for transfer of toner from a transfer belt to a medium such as paper, a value of electric current that varies with a change in electrical resistance, which depends on a type and water absorption of paper, is larger than a value of electric current that varies in accordance with an image to be printed. For this reason, a scheme that applies a constant current bias and achieves application of a desired electric current by correcting the value of electric current in accordance with an image to be printed is employed for the secondary transfer in a number of example configurations.
Meanwhile, an image forming apparatus has the following structural characteristic. A front surface, to which toner is to be transferred, of a medium such as paper has more pathways, e.g., a neutralizing brush, through which electric current flows than a back surface of the medium. Accordingly, electric discharge is more likely to occur on the front surface. For this reason, applying a secondary transfer bias to an intermediate transfer driving roller is generally considered as being advantageous.
In a case where an excessive amount of electric current should flow (discharge) though a medium such as paper, a voltage drop can result in a defective image or activation of a protective circuit in the image forming apparatus. Activation of the protective circuit in the image forming apparatus is equivalent to detection of an error, whereby operation of the image forming apparatus is stopped.
Meanwhile, in a small-size image forming apparatus, a distance between an intermediate transfer driving roller and a photosensitive element is small. Accordingly, when a secondary transfer bias is applied to the intermediate transfer driving roller, the applied bias can disadvantageously discharge to a nearby primary transfer roller.
The applied bias can discharge to a nearby photosensitive element; also in that case, a voltage drop can result in a defective image or activation of a protective circuit in the image forming apparatus. Activation of the protective circuit is equivalent to detection of an error, whereby operation of the image forming apparatus is stopped.
In the technique disclosed in Japanese Laid-open Patent Publication No. 2009-122168, the secondary transfer bias is applied to the secondary transfer roller side to avoid such a disadvantageous situation that the applied bias discharges to a nearby photosensitive element or a nearby primary transfer roller, with the knowledge that this configuration provides a disadvantage.
The disadvantage of the image forming apparatus according to Japanese Laid-open Patent Publication No. 2009-122168 is that the secondary transfer bias is likely to discharge or discharges through a medium such as paper.
In the light of the circumstances, there is a need for providing an image forming apparatus and an image forming method that can prevent or reduce discharge of a secondary transfer bias through a medium such as paper.
It is an object of the present invention to at least partially solve the problem in the conventional technology.